Meet the adventurous couple who has spent the past 4 years living in Airbnbs around the world for under $90 a night

They recently spent their 1,000th Airbnb night in Strasbourg,France. (Pictured: Debbie and Michael Campbell.) Michael and Debbie Campbell Since July 2013, retirees Michael and Debbie Campbell have lived in 150 different homes around the world, all booked through short-term rental service Airbnb.

On July 22, they spent their 1,000th night in an Airbnb in Strasbourg, France, the same country where they began their journey four years ago.

Throughout their Airbnb-hopping adventure, the Campbells have become a poster couple for the Silicon Valley-based travel company. They've visited Airbnb offices in Portland, San Francisco, and Dublin, spoken at a company conference in Paris, and penned a manual for Airbnb hosts called "Your Keys, Our Home." They've also mastered the skill of traveling on a budget, sticking to a $90 nightly rate and finding the best free and low-cost activities in each city they visit.

Business Insider recently caught up with Michael, 71, and Debbie, 61, to learn more about their ongoing trip around the world.

Scroll through to learn more about how they do it, and follow their adventure on their website, Senior Nomads.

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The Campbells' journey as self-proclaimed "Senior Nomads" began more than four and a half years ago when their daughter Mary was visiting them in Seattle for Christmas. Mary noticed a lengthy list of countries posted on the couple's fridge that they'd hoped to visit during their impending retirement.

Tel Aviv, Israel.
Courtesy of Michael and Debbie Campbell

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Mary presented an idea: go visit those countries, but instead of spending money on pricey hotels, try to find affordable short-term vacation rentals through Airbnb. Michael and Debbie were intrigued.

Girona, Spain.
Courtesy of Michael and Debbie Campbell

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Michael, a former sports promoter, and Debbie, a graphic designer, had always been travelers, even moving from the US to London for five years when their four children were young to experience life overseas.

After months of "crunching the numbers," the couple determined that Mary's idea could work. "[We realized that] if we took our Social Security and retirement funds and doled that out every month to travel and live in Airbnbs, we could spend the same amount we would spend in our rocking chairs in Seattle," Michael said.

Paris, France.
Courtesy of Michael and Debbie Campbell

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Michael and Debbie — who have been married for 38 years — sold their car and beloved sailboat, pared down their possessions, rented out their Seattle townhouse, and bought a round-trip ticket to Paris, France, where Mary lived with her young family.

Belgrade, Serbia.
Courtesy of Michael and Debbie Campbell

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From there, they would embark on a six-month journey around Europe, staying exclusively in Airbnbs with a nightly budget of around $90. After six months, they'd return to Seattle to reevaluate their scheme.

Visiting Airbnb offices.
Courtesy of Michael and Debbie Campbell

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But the Campbells surprised themselves and within three months decided to keep going. Four years later, Michael and Debbie are still touring the globe — and they haven't abandoned their budget. Michael calculated that their average nightly Airbnb rate over the past four years was $89.

Pezenas, France.
Courtesy of Michael and Debbie Campbell

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In fact, the pair is so meticulous with their spending that they've filled more than a dozen notebooks with receipts and logs of every purchase they've made. Luckily, their favorite mode of transportation — walking — is free. They've walked an average of 8,000 steps a day, a total distance that would equal walking from Seattle to New York and back again, says Michael.

Michael and Debbie also track their daily activities in the journals.
Courtesy Michael and Debbie Campbell

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Still, a strict budget doesn't hold them back. Michael is a sports fanatic whose choice activity in any city is attending the big game. Debbie, a lover of cooking and art, heads to the area's food markets and galleries. And together they take walking tours and look for free and affordable concerts.

Cuypmarkt in Amsterdam.
Courtesy of Michael and Debbie Campbell

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The couple rarely eats out, but doesn't "feel deprived," Michael said. "We try and live like locals ... we're shopping in the markets and doing things in our neighborhood," he said.

In Mexico City, the couple ate three meals a day for under $10.
Courtesy of Michael and Debbie Campbell

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The pair said continuous travel has been a mind shift. "We're not on vacation," Debbie said. "We're just living our daily lives in other people's homes in other countries." The Campbells have been to 68 countries — 20 in the last year alone — and more than 200 cities.

Havana, Cuba.
Courtesy of Michael and Debbie Campbell

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Michael and Debbie travel light — and they'd like to keep it that way. "If you can’t drink it, eat it, write on it, or attend it, then you can't buy it," Michael said. "It's very freeing," Debbie said.

Michael and Debbie Campbell

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By now, the Campbells are bona fide experts at booking Airbnbs. Their list of must-haves includes wifi, a large table for working, reading lights on both sides of the bed, and ideally, a location in the city center. They prefer to book an entire house or apartment to themselves rather than sharing — it feels more like home that way, they said.

Debbie returning from a grocery haul on the Mediterranean island of Malta.
Courtesy of Michael and Debbie Campbell

Michael and Debbie have a running list of every Airbnb home they've stayed in around the world. Though they say it's "not necessarily our recommendations," it shows what they look for in an Airbnb.

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"There's an art to picking an Airbnb," they said. "We read the reviews really carefully and try to ascertain from the reviews who's really a good host." They then contact about five potential hosts and determine the best match.

The couple's Airbnb in Havana, Cuba is one of their favorites so far.
Courtesy of Debbie and Michael Campbell

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"Our hope is that by the time we get there we have a friend who knows something about us and knows what we're interested in … someone who's doing Airbnb because they enjoy hosting people and hospitality," Michael said.

Michael and Debbie Campbell

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Two summers ago, Michael and Debbie cut ties at "home" and sold their Seattle townhouse, though two of their grown children still live in the area. They plan to keep traveling: "We will keep doing this as long as we are having fun, learning, staying close to our budget and we're still in love."

Michael and Debbie in Ephesus, Turkey.
Courtesy of Michael and Debbie Campbell

To see more photos and stories from Michael and Debbie's Airbnb journey around the world, check out their blog, Senior Nomads.